AM broadcast bands have, of course, been used for a long time in the radio field and did in fact dominate modern radio broadcasting for many years. Since the advent of AM broadcasting, however, FM radio broadcasting has become more and more in demand because of the concurrent development of FM stereo broadcasting systems. Until recently, there has been little or no effort involved in developing AM stereo systems. However, experience has shown that even though FM systems have some advantages as far as static reduction and the like, they also have difficulties in that the "line of sight" requirements severely limit the transmission distance that can be achieved by FM broadcasts. This is especially true in mountainous or hilly regions. Consequently, recent efforts have resulted in the development of various types of AM stereo broadcasting systems which operate on the standard AM broadcast band. In particular, there is now available an AM/PM stereo system which uses the standard AM broadcast band and also operates satisfactorily with existing AM transmitting systems with substantially no modifications required. Thus, this AM/PM stereo system holds great promise for future AM stereo broadcast and will especially be effective if the receiving system can be kept at a reasonable consumer price. Of course, to be completely accepted as an alternate to present FM stereo broadcast systems, it is believed that any AM substitute system will necessarily need to operate substantially similarly, so far as the consumer or listener is concerned, to the presently available FM stereo systems. Thus, as is the case with FM stereo broadcasting, there should also be available on an AM stereo receiving system the stereophonic broadcast indicator light to indicate the station is in fact broadcasting in stereo. Further, as was indicated hereinabove it is extremely desirable, considering the vast number of AM transmitters now in service, that these existing transmitting systems be capable of incorporating AM stereo without extensive modifications. It will also be appreciated that a simple and inexpensive technique for providing an AM stereo receiver may be necessary for consumer acceptance.
Therefore, it is an object of the present invention to provide a simple and inexpensive technique for decoding and providing stereo signals from AM/PM transmitted signals on a standard AM broadcast band.
It is still another object of this invention to provide both methods and apparatus to monitor and detect the existence of an AM stereo broadcast and provide a visual indication of such a broadcast.
It is yet another object of the present invention to provide methods and apparatus which do not require the expensive phase lock loop techniques of recovering PM phase modulated signal from a carrier frequency.